This invention concerns a process for forming an M-S (Metal-Semconductor) ohmic contact on lightly doped regions, which consists in particular of enriching the surface of the semiconductor on which the contact is to be formed, by means of ion implantation of a dopant and subsequent annealing.
A basic requirement of the M-S ohmic contains which act as an interface between a semiconductor device and a metal terminal is that the not introduce appreciable voltage drops when the current passes through them, so as not to alter the electrical properties of the device itself.
One of the main factors on which the specific contact resistance of an M-S junction depends, consists of the concentration of electrically active dopant in the semiconductor, in correspondence with the M-S interface, and of the choice of metal used.
If, when forming an M-S ohmic contact, the concentration of dopant active on the surface of the semiconductor is not sufficiently high, the surface is enriched with dopant. The most widely used enriching methods consist of pre-deposition and ion implantation of the dopant.
The first method calls for very high temperatures (over 900.degree. C.). With the second method, if the ion bombardment has produced a surface layer of amorphous material, it is necessary to carry out the subsequent annealing at a temperature higher than 500.degree.-550.degree. C. in the case of silicon, with a subsequent epitaxial regrowth in the solid phase ("SPE") of the damaged layer and activation of the implanted species. Whereas, if the ion bombardment has not caused amorphization, it is necessary to use a higher annealing temperature to activate the implanted species.
With both the first and the second method, the temperature at which the thermal processes are carried out is incompatible with the presence, on the front of the wafer, of metallization levels (for example, aluminum) or layers of organic material (for example, polymide).